This special internal format was defined in the WebGL 1 spec as a
special unsized format with at least 16 bits of depth and at least
8 bits of stencil. Intenally ANGLE will translate this to packed
24/8 depth/stencil.

The new test is adapted from the WebGL test:
conformance/renderbuffers/framebuffer-object-attachment

BUG=angleproject:1708

Change-Id: I44b03e41889eed02481f603b8d52c530dcfed5ce
Reviewed-on: https://chromium-review.googlesource.com/442094Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>